The bonding wire for connecting the electrodes on a semiconductor element to the external leads is conventionally a thin wire of gold alloy because of its good electrocoductivity, corrosion resistance, or other reliable properties.
Bonding of the thin gold alloy wire is typically performed by a thermocompression bonding process, in which the top end of a thin gold alloy wire is melted by heating with an electric torch to form a ball due to the surface tension of the melted metal, the ball is then pressed against an electrode on a semiconductor element heated at 150 to 300.degree. C. with an ultrasonic vibration applied thereto to form a bond, and thereafter, bonding to the external lead is performed by an ultrasonic compression bonding process in the same manner.
Advanced integration of semiconductor elements involves an increased number of electrodes on a semiconductor element. Miniaturization of semiconductor elements involves a reduced area of an electrode and a shortened pitch or distance between electrodes, and therefore, requires that bonding wires should provide a reduced ball size and should have a reduced diameter. The reduction in the ball size reduces the heat input from an electric torch to the bonding wire upon forming the ball, thereby reducing the length of the region of the wire in which recrystallization occurs during the formation of the ball. It is known that there is a close relationship between the recrystallization region length and the loop height of the wire, such that a bonding wire having a long recrystallization region has a high loop, and vice versa. Namely, under a reduced ball size, the conventional loop height cannot be obtained unless the recrystallization region length is increased by decreasing the contents of the alloying elements in a bonding wire gold alloy that control the recrystallization. The alloying elements that control the recrystallization generally also improve the mechanical strength of wire, so that reduction in the contents of such alloying elements causes reduction in the mechanical strength of wire.
The loop height is also reduced by thinning of a bonding wire. Specifically, the ball size is usually controlled to a fixed value within the range of from 2.5 to 3.0 times the wire diameter, so that reduction of the wire diameter brings about the same effect as that obtained by reducing the ball size, and therefore, thinning of a bonding wire causes reduction in the loop height.
The reduction in the ball size, or the thinning of the bonding wire, thus leads to reduction in the loop height, and thereby, causes the problems in reliability and production yield of semiconductor devices because the bonding wires are undesirably brought into contact with the semiconductor element or the die pads of leadframes, causing device malfunction or defects, because the pull strength is reduced by the reduced loop height, and because bonding wires are brought into contact with each other by a wire flow induced by wire deformation because of a flow resistance to a resin used for sealing the semiconductor elements.
To prevent occurrence of these problems, a usual measure is to reduce the contents of the elements that control the recrystallization region length. However, this causes a reduction in the mechanical strength and coarsening of the recrystallized grains of a wire in the portion immediately above the ball being formed, so that the portion immediately above the ball is easily broken by vibration during handling after bonding, to cause poor reliability and a reduced production yield of semiconductor devices.
To solve this problem, many kinds of thin gold alloy wires have been proposed; for example, a gold bonding wire comprising from 5 to 100 ppm by weight of calcium (Japanese Unexamined Patent Publication (Kokai) No. 53-105968), a gold bonding wire comprising from 3 to 5 ppm by weight of calcium, from 1 to 8 ppm by weight of beryllium, and/or from 5 to 50 ppm by weight of germanium (Japanese Unexamined Patent Publication (Kokai) No. 53-112060), and a gold bonding wire comprising from 1 to 8 ppm by weight of beryllium (Japanese Unexamined Patent Publication (Kokai) No. 53-112059).
However, these gold bonding wires have the problem that the loop height after bonding is reduced because of an increased content of the alloying elements for preventing coarsening of the crystal grains immediately above the ball. Moreover, the alloy composition containing beryllium alone as an additive alloying element to the gold base provides a high loop but has the problems that the mechanical strength of wire is lower and the wire flow after resin molding is more significant in comparison with the other alloy compositions.
Control of the loop height is currently effected by the loop shape control system of a bonder to ensure the necessary loop height, but this requires a long duration of time for bonding and causes problems somewhat in the productivity. The conventional bonding wire is not successful in providing large loop height while preventing the coarsening of crystal grains at the wire portion immediately above the ball. There has been a strong need for a thin wire of gold alloy for wire bonding that reliably provides a large loop height and high bonding strength.
The present inventors studied many of the conventionally proposed thin wires of gold alloy for wire bonding and discovered the following fact. The conventional thin wires of gold alloy for wire bonding have a pull strength greater than that of a pure gold wire containing no additive elements, but when a high loop must be formed, the contents of the alloying elements must be reduced, with the result that the pull strength becomes poor and the recrystallized crystal grains easily coarsen in the wire portion immediately above the ball thereby causing damage in the ball neck during the bonding operation. Reduction in the wire diameter involves reduction in the pull strength in accordance with the reduced wire sectional area. Therefore, to ensure a good pull strength, the contents of the alloying elements must be increased, with the result that the large loop height cannot be provided.